The present invention relates to a remote transmitting and receiving apparatus for receiving a signal, which is received by a key transmitting and receiving apparatus such as telephones, at a position distanced from said key transmitting and receiving apparatus and for transmitting said signal at a position distanced from said key transmitting and receiving apparatus through said apparatus, and more specifically, to a cordless transmitting and receiving apparatus which uses a commercial power line in a transmission line of signal to the key transmitting and receiving apparatus and can provide transmitting and receiving between said cordless transmitting and receiving apparatus and the key transmitting and receiving apparatus without use of a communication exclusive-use cord.
In the past, various remote transmitting and receiving apparatuses have been proposed wherein in a room separately from a room where a telephone is set, an aural signal is transmitted to an external telephone line through said telephone or a signal transmitted to said telephone is received. In prior art remote transmitting and receiving apparatus, the signal from the telephone is transmitted by making use of an in-door commercial power line and the signal to the telephone is transmitted wirelessly from antennas provided on the remote transmitting and receiving apparatus and the telephone, respectively, and such a conventional apparatus is widely called a wireless transmitting and receiving apparatus because it has not exclusive-use signal cord.
One embodiment of the prior art wireless transmitting and receiving apparatus as described will be explained with reference to FIG. 1. A remote transmitting and receiving apparatus 11 comprises a receiving station and a transmitting station, said receiving station comprising a microphone 12 for entry of aural signal, a transmission circuit 13 input of which is connected to output of said microphone 12 and having a modulation circuit, an amplification circuit, etc., and an antenna 14 connected to output of said transmission circuit 13 to transmit an output signal thereof, said receiving station comprising a loop antenna 16 electromagnetically coupled to an AC line 15 of a commertial power supply for in-door wiring, a receiving circuit 17 input of which is connected to the loop antenna 16 and having a frequency conversion circuit, an intermediate frequency amplification circuit, a detection circuit, etc., and a speaker 18 connected to output of the receiving circuit 17 to put out a received signal as an aural signal. On the other hand, a key transmitting and receiving apparatus 21 such as a telephone comprises a receiving station and a transmitting station, said receiving station comprising an antenna 22 for receiving a signal from the antenna 14 of the remote transmitting and receiving apparatus 11 and a receiving circuit 23 similar to the aforesaid receiving circuit 17, of which input is connected to said antenna 22, said transmitting station comprising a transmission circuit 24 similar to the aforesaid transmission circuit 13, a transformer 25 for transmission output, of which input is connected to output of said transmission circuit 24, and a coupling capacitor 26 connected to output of said transformer 25. Output of the coupling capacitor 26 is connected to a transmitting and receiving plug 27, which is in turn connected to a convenience 28 for AC power supply in a room where the key transmitting and receiving apparatus 21 is installed. The convenience 28 is connected to an AC line 15. The key transmitting and receiving apparatus 21 is provided with a signal shunt 30 including a hybrid coil, which supplies a received signal from the receiving circuit 23 to an external telephone line 29 and supplies a signal sent from the telephone line 29 to the transmitting circuit 24.
The aural signal from the microphone 12 of the remote transmitting and receiving apparatus 11 is transmitted by the transmission circuit 13 through the antenna 14 while carrying it on, for example, a carrier wave of 49 MHz, and said signal is received by the antenna 22 of the key transmitting and receiving apparatus 21. The signal received by the antenna 22 is subjected to the required signal processing at the receiving circuit 23, after which it is sent to the external telephone line 29 through the signal shunt 30. On the other hand, the aural signal transmitted from the telephone line 29 is supplied to the transmission circuit 24 via the signal shunt 30. In the transmission circuit 24, said signal is carried on, for example, carrier wave of 1.7 MHz, and supplied to the AC line 15 through the coupling capacitor 26. The transmission signal carried on the AC line 15 is subjected to electromagnetic induction by the loop antenna 16 of the remote transmitting and receiving apparatus 11, and a wave containing the thus guided voice is subjected to the required receiving processing in the receiving circuit 17, after which it is released by the speaker 18. In this manner, the transmission from the remote transmitting and receiving apparatus 11 to the key transmitting and receiving apparatus 21 is effected by the transmission of the wave by the antenna 22, whereas the transission to the key transmitting and receiving apparatus 21 is effected through the AC line 15.
In the prior art wireless transmitting and receiving apparatus as described above, a carrier-frequency signal from the remote transmitting and receiving apparatus 11 to the key transmitting and receiving apparatus 21 is 49 MHz of VHF (very high frequency) band as previously mentioned, and if said signal is intended to be used for transmission and receiving between rooms which are partitioned by a number of walls and floors in buildings or the like, the transmission efficiency is considerably decreased due to the presence of these walls and floors. On the other hand, the use of a carrier-frequency signal of approximately 1.7 MHz of MF (medium frequency) band which not largely affects on the transmission efficiency can be contemplated, but to this end, the length of the antennas 14 and 22 must be extended greatly. In the key transmitting and receiving apparatus 21 which is the fixed station, it is not inconvenient to extend the length of the antenna 22 thereof, but in the remote transmitting and receiving apparatus 11, the antenna is preferably of the handy type in terms of the nature thereof. Thus, the lengthy antenna is inconvenient to carry for such a handy type transmitting and receiving apparatus.